

Hi,

That's actually a question I don't have good answer myself. 

For a long time, LinuxCNC would run only with i386, but nowadays I think it has changed

 and amd64 could be used as well, but the safe bet is to go with i386:

http://cdimage.debian.org/cdimage/archive/7.11.0/i386/iso-cd/debian-7.11.0-i386-netinst.iso